Amplification using amplitude reconstruction of amplitude and/or angle modulated carrier

ABSTRACT

The present invention provides high power linear amplification of an amplitude and/or phase modulated signal using multiple saturated (or if desired, unsaturated) amplifiers driven by an appropriate set of switched, and/or phase modulated constant amplitude signals derived from the input signal. The present invention combines three amplitude reconstruction techniques and implements the amplitude reconstruction modulator digitally.

FIELD OF THE INVENTION

The present invention relates to the field of signal amplification; moreparticularly, the present invention relates to high power, lowdistortion amplification of an input comprising a single amplitudemodulated carrier or a composite signal comprising multiple modulatedcarriers.

BACKGROUND OF THE INVENTION

A need exists to amplify signals comprising a single modulated carrieror a composite signal comprising several modulated carriers to highpower levels for use in wireless communication base stations. Thecharacteristics of the input signal require a high degree of linearityto substantially reduce and in some cases minimize distortion artifactsfrom appearing at the amplified output. In the RF frequency domain,standard high power amplifiers typically do not possess sufficientlinearity to amplify either the single amplitude modulated RF signalsnor the composite multiple RF carrier signals to meet stringent wirelessbase station requirements. Non-linear distortion products from suchamplifiers can occur at the output in the form of spectral spreading orspectral regrowth of the modulated carriers or in spurious in bandintermodulation products in the case of multiple RF carriers.

Currently, several techniques are used to compensate for high poweramplifier non-linearity. For example, feedforward cancellation is aclosed loop technique that introduces a compensation RF component intothe output that cancels the nonlinear distortion products created by theamplifier. An example of a feedforward cancellation technique isdescribed in U.S. Pat. No. 5,528,196, entitled "Linear RF AmplifierHaving Reduced Intermodulation" (Baskum, et. al.), issued Jun. 18, 1996.Another technique is referred to as pre-distortion, which is a method topre-distort the phase and amplitude of the input signal in a manner thatcounteracts and compensates for amplifier nonlinarity. The combinationof the predistortion circuit and the nonlinear amplifier results in anet linear amplification. Examples of such an amplification techniqueare shown in U.S. Pat. No. 4,462,001, entitled "Baseband Linearizer forWideband, High Power, Nonlinear Amplifiers" (Girard), issued Jul. 24,1984 and U.S. Pat. No. 5,576,660, entitled "Broadband PredistortionLinearizer With Automatic Temperature Compensation For MicrowaveAmplifiers" (Pouysegur et. al.), issued Nov. 19, 1996.

Other prior art techniques are based on indirect methods of amplitudereconstruction. These methods generally separate the input signal intothe amplitude and angle modulation components. The constant amplitudeangle modulation component is easily amplified using nonlinearamplifiers. The amplitude modulation component is reintroduced into theoutput using one of several methods. In one method referred to asswitched amplifiers, multiple power amplifiers are employed which arecapable of being individually switched on and off to vary the powerdelivered to the load. In this method, the RF drive levels are switchedon or off such that the number of active amplifiers is proportional tothe signal envelope, or amplitude. When the multiple amplifier outputsare combined, the result is a signal whose envelope approximates that ofthe input signal. An example of this technique is shown in U.S. Pat. No.5,132,637, entitled "RF Power Amplifier System Having ImprovedDistortion Reduction" (Swanson), issued Jul. 21, 1992. In anothertechnique referred to as pulse modulation, the angle modulated componentis further modulated with a pulse waveform prior to amplification. Thepulse modulation frequency is selected to be much higher than theoperating bandwidth of the input signal, and the duty cycle of the pulsewaveform is adjusted to be proportional to the envelope modulation. Theangle modulation which exists on the original signal is unaffected, butwhen the amplifier output is suitably band pass filtered, the originalamplitude modulation is reintroduced onto the carrier waveform. Anexample of this technique is shown in U.S. Pat. No. 5,249,201, entitled"Transmission of Multiple Carrier Signals in a Nonlinear System"(Posner), issued Sep. 29, 1991. In a third technique referred to asphase modulation, a pair of high power amplifiers is required. The anglemodulated carrier driving each amplifier is further separately anddifferentially phase modulated as a function of envelope signal. Phasemodulation is introduced in a manner such that when the two amplifieroutput signals are combined, the imparted differential phasing causesthe result to be amplitude modulated. An example of this technique isshown in U.S. Pat. No. 4,178,557, entitled "Linear Amplification WithNonlinear Devices" (P. Henry), issued Dec. 11, 1979.

In all of the latter three methods, a key element is the operation ofthe power amplifier(s) in a saturated or nearly saturated mode. Thisallows highly nonlinear amplifiers (such as Class C) to be used and alsoresults in efficient power generation. In all cases, the result islinear amplification of the input signal. The degree of linearperformance actually achieved generally depends on the precision of theimplementation. Typically, linear dynamic range has been limited to 40dB to 60 dB. Some systems, such as GSM, will require dynamic ranges onthe order of 74 dB. Thus, a new approach is required to meet thisrequirement for very wide dynamic range.

SUMMARY OF THE INVENTION

A method and apparatus for amplifying a signal is described. In oneembodiment, the present invention comprises a method and apparatus forseparating an input signal into an amplitude modulation component and anangle modulation component, dividing the angle modulation component intomultiple paths having amplifier stages, inducing amplitude modulationinto the plurality of paths by performing amplifier switching, pulseduty cycle modulation and phase modulation, and combining outputs of theamplifier stages in each path to create an output signal. At least twoof the parallel paths have a switch and/or a phase modulator precedingthe amplifier stage of the path.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 is a block diagram of an amplifier performing a signalamplification and signal amplitude reconstruction process.

FIGS. 2a and 2b illustrate duty cycle and phase modulation for amplitudereconstruction.

FIG. 3 illustrates AM/AM and AM/PM characteristics on an idealamplifier.

FIG. 4 illustrates AM/AM and AM/PM characteristics for an amplifier withphase and amplitude imbalance plotted vs. input envelope amplitude.

FIG. 5 illustrates amplitude and phase characteristics for an amplifierwith phase and amplitude imbalance plotted vs. amplitude reconstructionphase modulation.

FIG. 6 is a block diagram of one embodiment of an amplifier using adigital signal processor.

FIG. 7 illustrates the frequency plan for the input and calibrationsignals through the output of a digital to analog converter.

FIG. 8 is a block diagram of a digital processor.

FIG. 9 illustrates the frequency plan for processing within a digitaldown converter.

FIG. 10 illustrates the frequency plan for processing within a digitalup converter and conversion by a digital to analog converter.

DETAILED DESCRIPTION

A linear amplification technique is described. In the followingdescription, numerous details are set forth, such as numbers ofamplifiers, specific frequencies, numbers of adders, etc. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

The amplification technique provides high power linear amplification ofan amplitude and/or phase modulated signal using multiple saturated (orif desired, unsaturated) amplifiers driven by an appropriate set ofswitched and/or phase modulated constant amplitude signals derived fromthe input signal. In one embodiment, the linear amplification techniquecombines three amplitude reconstruction techniques and implements theamplitude reconstruction modulator digitally. The combination of threeindependent techniques significantly increases the achievable dynamic orconversely reduces the precision required of any one of the modulationtechniques.

Referring to FIG. 1, one embodiment of the high power linear amplifiercomprise signal separator 51 to separate the input signalS_(in).spsb.(t) 14 into a phase, or angle, modulation component and anenvelope, or amplitude, component. The input signal may comprise asingle or multiple carrier signals. In one embodiment, signal separator51 comprises (hard) limiter 1 and envelope detector 2. Other mechanisms(e.g., devices, functional units, circuits, etc.) to separate an inputsignal into angle modulation and envelope may be used.

A power divider 3 is coupled to receive the angle modulation componentand divides the angle modulation component into the multiple, identicalparallel paths. Multiple signal processing modules 4 are coupled toreceive outputs a₁ (t)-a_(N) (t). In one embodiment, each of signalprocessing modules 4 comprises series switch 5, phase modulator (phaseshifter) 6 and power amplifier 7 (i.e., an amplifier stage). In oneembodiment, amplifiers 7 may comprise Class C amplifiers or other fullysaturated RF amplifiers. The use of these amplifiers achieves linearamplification even though the amplifiers are not linear.

Amplitude reconstruction modulator 11 is coupled to signal separator 51,signal processing modules 4, and an optional adaptive calibration unit13 and provides modulation signals to signal processing modules 4. Notethat depending on the application, series switch 4 and phase modulator 6can be used separately or together. That is, these elements may beapplied individually.

Power combiner 8 combines outputs d₁ (t)-d_(N) (t) of the parallelamplifiers 7 of signal processing modules 4. Output protection andfiltering is provided by isolator 9, which is coupled to the output ofpower combiner 8 and output filter 10, which is coupled to the output ofisolator 9.

Additional units comprising an adaptive equalizer controller 12,equalizer filters 26, and the adaptive calibration unit 13 may beincluded, if required, to meet performance requirements imposed on theamplifier.

In operation, a composite input signal, S_(in) (t), 14 which comprisessingle or multiple carriers, is separated into two paths, one containingonly the envelope modulation component and the second containing onlythe angle modulation component. The angle modulation component, B_(in)(t), 15 is obtained from limiter 1 (or some similar device).

Signal 15 is divided multiple (N) ways to provide a set of drivesignals, a_(k) (t), k=1, 2, . . . N, 17 for each of N parallel signalprocessing modules 4.

Depending on the application, series switch 5 and/or phase modulator 6can be used either separately, or in combination, depending onrequirements for dynamic range, thereby allowing the dynamic range to beextended beyond that which is attainable by each of the techniques usedabove. Note that use of techniques in the amplifier described hereinimproves the efficiency of the amplifier.

Switch 5 allows amplifier switching and/or imparts a variable pulsewidth modulation (i.e. the duty cycle modulation), e_(k) (t), 24 at asuitable sample rate on drive signal 17. Switch 5 is not necessary formany applications and is optional. The phase modulator 6 imparts avariable phase modulation, f_(k) (t), 25 on drive signal 17. Theresulting modulated drive signal, c_(k) (t), 19 output from phasemodulator 6 drives amplifier 7 at or near saturation. Drive signals 17representing the phase component of the input signal, being a constantamplitude, normally drive each amplifier into or near saturation.

The signal envelope or amplitude modulation, obtained from envelopedetector 2, is used to reconstruct the amplitude information of inputsignal 14 by controlling the state of each switch 5 and/or phasemodulator 6 contained in signal processing modules 4. A combination ofamplifier switching, pulse duty cycle modulation and/or phase modulationintroduced into each path is used to induce amplitude modulation toappear at the output of power combiner 8, which is used to sum theoutputs of the N paths. This amplitude modulation at the output ofcombiner 8 matches the amplitude modulation of input signal 14. Theamplifier switching, pulse duty cycle and phase modulation areintroduced in such a way as to not induce additional phase modulation tothe output signal 23. Thus, the amplitude and phase modulation inducedto appear at the output match the amplitude and phase modulation of theinput signal.

Amplitude reconstruction modulator 11 provides independent control ofeach of signal processing modules 4. These N independent or similar dutycycle modulation signals 24 and/or phase modulation signals 25 are usedto reintroduce amplitude modulation on output signal 23 proportional toinput amplitude modulation component (A_(in) (t)) 16 of input signal 14output from detector 2. Thus, in one embodiment, amplitudereconstruction is accomplished using duty cycle and/or phase modulationimplemented via series switches and phase modulators.

Power combiner 8 performs a vector summation of amplifier output signalsd_(k) (t) 20. The magnitude of signal 21 at the output, U.sub.(t), ofcombiner 8 is dependent on the phase and amplitude of the modulatedsignals 20 output from amplifier 7 of each of signal processing modules4. Ideally, the amplitudes of the signals 20 output from amplifier 7 ofeach of signal processing modules 4 are equal, making output signal 21dependent only on the number of amplifiers activated, the duty factor ofthe pulse modulation and the added phase modulation 25.

Isolator 9 provides matched impedance for each amplifier 7 in signalprocessing modules 4 and additionally absorbs out of band spectralsidelobes introduced by duty cycle modulation 24 generated by eachswitch 5 in signal processing modules 4. Filter 10 performs a bandlimiting filtering operation to pass the central frequency components ofsignal, (V(t)), 22 while rejecting the spectral sidebands introduced byeach switch 5 as duty cycle modulation 24 and/or introduced by eachphase modulator 6 as phase modulation 25. Filter 10 further impartsadditional amplitude modulation on its output signal, (W(t)), 23 byconverting the duty cycle associated with duty cycle modulation 24 intoamplitude modulation.

In one embodiment, an optional adaptive amplitude and phase calibrationmodule 13 is coupled to compare output signal 21 of combiner 8 against adelayed version of input signal 14 (delayed so that the output and inputcomparison is made for the same instant of time). This comparisonproduces calibration signals which are driven to amplitudereconstruction modulator 11 and used for compensation of amplitude andphase imbalances that exist among the N parallel paths containing signalprocessing modules 4. The difference between the delayed input and theoutput of combiner 8 is driven to zero by adjusting the values used forthe duty cycle and/or phase modulators.

Adaptive equalizer controller 12 and equalizer filters 26 may beincluded to equalize amplitude and phase variations between thechannels. In other words, the use of equalizers may remove frequencyresponse imbalances between channels.

FIGS. 2a and 2b illustrate the amplitude reconstruction modulationperformed by one embodiment of amplitude reconstruction modulator 11.Referring to FIG. 2a, pulse modulation waveform 27 is defined for theamplitude reconstruction pulse modulation applied to each switch 5 ofsignal processing modules 4. Pulses of variable width T_(pw) 29 occur ata fixed interval T_(pri) 28. The frequency associated with T_(pri) 28 isselected to be sufficient to place spurious spectral componentsintroduced by the pulse modulation outside the bandwidth of filter 10.Filter 10 removes the spurious spectral components and produces signal23 which has an amplitude proportional to the duty cycle of waveform 27.

Additionally, switch 5 in each of signal processing modules 4 can beindividually controlled by amplitude reconstruction modulator 11 toimplement amplifier switching. In this case, the number of signalprocessing modules 4 activated, and consequently the number ofamplifiers 7 turned on, is proportional to amplitude modulationcomponent 16.

Referring to FIG. 2b, phase modulation is defined for the amplitudereconstruction phase modulation applied to pairs of signal processingmodules 4. The signals passing through a pair of signal processingmodules 4 are further modulated by the complex phasors 30 and 31. Letφ_(k) and φ_(k+1) be the phase shift applied to two of the signalprocessing modules 4. Phasors 30 and 31 represent the phase shiftapplied to the signals passing through signal processing modules 4. Theresultant signal appearing at the output of combiner 8 is the vector sumof the original signal modified by resultant phasor 32. By controllingφ_(k) and φ_(k+1), considerable freedom exists to control both the phaseand amplitude of phasor 32. When the amplifiers 7 in signal processingmodules 4 are balanced, operating phases φ_(k) and φ_(k+1)differentially (i.e. φ_(k+1) =-φ_(k)) amplitude of output 32, B=2 cos(f)33. When equal phase shifts are employed (i.e. φ_(k+1) =φ_(k)), then thenet phase of φ_(k) or φ_(k+1) is imposed on output signal 21. Ingeneral, any phase pair combination can be decomposed into odd(differential) and even (equal) phase pairs. In a similar manner, otherpairs of signal processing modules 4 produce similar results. The totaloutput from combiner 8, therefore, is the vector sum of all the Nphasors to produce the desired amplitude and/or phase modulation. Oncecalibration has achieved amplifier balance (phase and amplitude)amplitude modulation reconstruction is implemented using the relationφ_(k) (m)=-φ_(k+1) (t)=arccos (A (t)/A_(clip)) where A_(clip) is theclip level (maximum power output from the amplifier) for the amplitudemodulated signal.

The preceding discussion assumes that all channels are balanced in bothamplitude and phase. In normal operation, this is not the case andcalibration is necessary. In one embodiment, the amplifier employs acalibration process supplies a fixed amplitude and phase correction toeach channel. A technique for calibration is suggested in FIG. 3, FIG. 4and FIG. 5.

Referring to FIG. 3, there is shown the amplitude modulation toamplitude modulation (AM/AM) and amplitude modulation to phasemodulation (AM/PM) characteristics, which should exist for an idealbalanced amplifier configuration. Here, the output amplitude, A_(out),is linearly related to the input amplitude, A_(in), up to the amplitudeclip level, A_(clip). The phase difference, F_(D), between the outputand input waveform is constant (zero in this case).

Referring to FIG. 4, there is shown an exemplary AM/AM and AM/PMcharacteristic for a specific amplitude and phase imbalance (Case 1defined below). Let A1and A2 refer to the amplitudes of the signalsappearing at the input of a two-channel combiner. Let Φ₁ and Φ₂ be thephases of the two signals attributable to the two paths. Then letΦ.sub.Δ =Φ₁ -Φ₂ be differential phase of the two paths. Then the fourcases are: Case 1 (A₁ >A₂ &Φ.sub.Δ <0), Case 2 (A₁ <A₂ &Φ.sub.Δ <0),Case 3 (A₁ >A₂ &Φ.sub.Δ >0), and Case 4 (A₁ <A₂ &Φ.sub.Δ >0). In allcases, an amplitude imbalance produces a situation where the outputamplitude cannot reach zero value. Instead a minimum is reached whosedepth and location depends on the amplitude and phase imbalance. Thiscan be explained by referring to FIG. 2b. If the two phasors arerepresented by A₁ exp.sup.(jφ.sbsp.1.sup.). and A₂exp.sup.(jφ.sbsp.2.sup.) respectively, then the resultant B 32 is givenby B=A₁ e.sup.(jφ.sbsp.1.sup.) +A₂ e.sup.(jφ.sbsp.2.sup.). The magnitudeimbalance can be represented by a differential quantity Δ, such that A₁=(1+Δ)A and A₂ =(1-Δ)A where A is the nominal ideal magnitude. The phaseimbalance can similarly be represented as a phase difference φ₀ suchthat φ₁ =φ₀ +b and φ₂ =φ₀ -B where B is the amplitude reconstitutionphase modulation. When these expressions in the preceding expression forthe resultant phasor, the magnitude |B| becomes sqrt(4 cos2(φ₀ +b)+4Δ2sin(φ₀ +B)). This function reaches a minimum at B=90 deg-φ₀ and hasvalue 2Δ.

FIG. 5 illustrates a data equivalent to FIG. 4 except that outputamplitude and differential phase is plotted against reconstruction phaseP_(mod). Estimates for the differential amplitude and phase imbalancecan be obtained from such curves. The location of the null and thecharacteristics of the differential phase in the vicinity of the nullindicates which of the four defined cases exist. Data needed toconstruct curves shown in FIG. 5 can be obtained directly from inputsignal 14 and from samples of output signal 21 during normal operation,thus eliminating the need for a special calibration cycle. In this case,when performing the calibration, two buffers store samples of the inputsignal and samples of the output signal being feedback. Input samplesare sorted by amplitude and indexed based on their amplitude. Using theordering and indexing of the samples of the input signal, the outputsamples are reordered. This allows for determining output sampleamplitude versus input sample amplitude. The same steps may be takenwith respect to plotting the phase difference between the input andoutput signals. Using the plots, estimates of the amplitude and phaseimbalance may be obtained from the detected minimum. Once estimates ofthe amplitude and phase imbalance are obtained, phase and amplitudecorrections can be applied to null both signals. As described below,phase and amplitude calibration signals are added to the phase 65 andamplitude 66 control signals shown in FIG. 8. In one embodiment, theprocess to null both signals is iterative in that the results ofapplying calibration signals to null both signals, a residual error isgenerated, requiring further calibration. The calibration process isthen repeated. In one embodiment, the calibration does not attempt tonull the entire error during a single calibration cycle. Instead, apercentage (e.g., one-half) of the error is iteratively corrected so asto migrate towards a reduction in the residual error. This helpsmaintain stability.

FIG. 6 is a block diagram of an amplifier using a digital signalprocessor to implement the amplitude reconstruction modulator. This highpower amplifier may be used as part of a transmitter in a communicationsystem (e.g., a wireless communication system). Referring to FIG. 6, theinput RF frequency signal and a signal from common local oscillator (LO)35 are coupled to inputs of channel mixer 36, which converts the inputRF frequency signal to an IF frequency range signal suitable for directconversion by analog to digital converter (ADC) 43, after undergoingfiltering by filter 42 to remove spurious signals resulting from themixing operation.

The output of ADC 43 is coupled to the input of digital signal processor44. Digital signal processor 44 performs amplitude reconstructionmodulation. The resulting signals output from digital signal processor44 drive amplifier channels that include amplifier channel modules 37.Each of the amplifier channel modules 37 comprises a digital to analogconverter 38 which performs digital to analog conversion on a outputfrom digital signal processor 44. The signal output from DAC 38 is mixedwith the local oscillator frequency from local oscillator 35 using mixer39. The output of mixer 39 is coupled to the input of filter 40, whichfilters the results of the mixing operation. The filter 40 outputs adrive signal to an amplifier 41.

The outputs of each of the amplifier channel modules 37 are coupled tothe inputs of power combiner 45. Power combiner 45 combines multipleamplifier outputs via vector recombination. In one embodiment, powercombiner 45 comprises a transmitter output. The results of the combiningperformed by power combiner 45 are input to isolator 46. The output ofisolator 46 is coupled to the input of filter 47. Power combiner 45,isolator 46 and filter 47 operate in the same manner as theircounterparts in FIG. 1.

The output of power combiner 45 is also fed back to a calibrationreceiver module 48 that comprises a mixer 70 that mixes the output ofpower combiner 45 with the frequency from local oscillator 45. Theresults of the mixing operation are filtered using a filter 71 and thenundergo analog digital conversion using ADC 72. The signal output fromADC 72 is input to digital signal processor 44.

FIG. 7 illustrates an exemplary frequency plan for one such realization.FIG. 7a illustrates the desired input frequency band centered at 1947.5MHz and a LO at 1887.5 MHz. FIG. 7b illustrates the spectrum after beingtranslated to a 60 MHz IF along with harmonics of the ADC 26.67 MHzsample rate. FIG. 7c illustrates the resulting spectrum after sampling.Note that subharmonic sampling has been employed in this example thatallows the sample frequency to be lower than the IF frequency.

FIG. 8 is a block diagram of one embodiment of the digital signalprocessing. FIG. 9 illustrates additional detail of the frequency plan,incorporated into the digital signal processor. Referring to FIG. 8, theoutput of ADC 43 is coupled to the input of digital down converter (DDC)55, which translates the signal frequency 75, by one quarter of the ADCsample rate (i.e. Fs/4) of ADC 43 to baseband 76, using a complexfrequency translation. DDC 55 also filters this signal to remove theundesired harmonic component at Fs/2 77, and the output samples aredecimated by a factor of two. The decimation reduces the sample rate.Thus, DDC 55 converts sampled real signals into complex baseband signalsand decimates the complex baseband signals. The output of DDC 55comprises an in-phase (I) component and a quadrature-phase (Q)component. FIG. 9d shows the resulting signal spectrum.

Input equalizer 56 equalizes amplitude and phase variations that mayexist and which are common to all channels. In one embodiment, inputequalizer 56 comprises a finite impulse response (FIR) filter thatfilters based on an equalizer filter coefficient generated by adaptiveequalizer FIR coefficient generation module 63 in a manner well-known inthe art.

Rectangular to Polar converter (R2P) 57 converts the rectangularcoordinate in-phase (I) and quadrature-phase (Q) input to polarcoordinate amplitude and phase format. The phase component representsthe angle modulation component of the input signal while amplitudecomponent represents the envelope component of the input signal.

The phase component is coupled to the input of phase modulation anddistribution module 59. Phase modulation module 59 is also coupled toreceive inputs from signal amplitude reconstruction phase modulationgeneration module 58 and optionally from adaptive phase and amplitudecalibration signal generation module 62. In one embodiment, signalamplitude reconstruction module 58 provides phase modulation P_(mod) 64which is supplied to the two adders 596a and 59b contained in phasecombining module 59. Note that P_(mod) is added with adder 59a, whileP_(mod) is subtracted with adder 59b. Phase calibration signals 65a-dare added to the phase signal in each channel using adders 59c-f,respectively. The outputs of phase modulation module 59 are coupled toone input of polar to rectangular converters (P2R) 60. Amplitudecalibration signals 66a-d from adaptive calibration module 62 are alsoapplied to another input of each of converters 60. Amplitude calibrationsignals 66c-d are generated by adaptive calibration module 62.

P2R 60 converts the polar coordinate amplitude and phase input signals(the amplifier channel signals) into rectangular in-phase (I) andquadrature-phase (Q) signals. The outputs of each P2R 60 are coupled toinputs of output equalizers 54a-d, which equalize amplitude and phasevariations that may exist. In one embodiment, equalizers 54c-d compriseFIR filters that operate based on equalizer coefficients generated byadaptive frequency equalization module 65 in a manner well-known in theart.

Transmit I and Q signals for each transmit channel are converted fromcomplex baseband signals to real signals and interpolated by a DigitalUp Converter (DUC) 61 located in each transmit channel, and then thedigital signals are converted to analog format by Digital-to-AnalogConverters (DAC) 38. These outputs drive the amplifier channels.

It should be noted that the embodiment described above advantageouslyincludes no multipliers. Other embodiments may include multipliers.

FIG. 10 illustrates a frequency plan for frequency conversion containedwithin the transmit channels. FIG. 10a shows the frequency plan presentat the input of DUC 61. In one embodiment, DUC 61 first interpolates thesignal by inserting zeros between samples to increase the sample rate,then filters the signal to remove the component at the new Fs/2.Finally, the signal is quarter band up shifted (i.e. Fs/4). Only thereal part is kept to produce spectrum shown in FIG. 10c.

One embodiment of a calibration procedure follows. This procedure may beimplemented in software, such that which runs on a digital signal,general purpose or dedicated processor, or with hardware (or acombination of both). First, referring to FIG. 1, a block of K samplesof input S_(in) (t) and output signals U(t) are obtained. This may beaccomplished by obtaining samples from ADCs 43 and 72 of FIG. 6. Let theblock of sampled digitized signals be noted by S_(in) (n) and U(n) forn=0, 1, 2, . . . , K-1.

Next each set of signals is decomposed into their complex form:

    S.sub.in (n)=A.sub.in (n) exp(jB.sub.in (n))

    U(n)=A.sub.out (n) exp(jB.sub.out (n))

Note that this form is automatically provided by digital signalprocessor 44 and amplifier modules 37.

Using these signals, Sin is adjusted to account for the time delaythrough digital signal processor 44 and amplifier modules 37: ##EQU1##where p is the known or estimated time delay.

The data contained in the data blocks is arranged to obtain plotssimilar to those in FIG. 4 or FIG. 5. These plots are point by pointplots of the corresponding delay adjusted samples (e.g., S_(in) (n),U(n)). Note that:

    A.sub.in =Magnitude(E.sub.in)

    A.sub.out =Magnitude(E.sub.out)

    B.sub.out -B.sub.in =Phase(E.sub.out /E.sub.in)

corresponds to the labels used in FIG. 4 and FIG. 5. This procedure maybe facilitated by first sorting A_(in) (n) in increasing order and thensorting the remaining variables (e.g., B_(in) (n), A_(out) (n), andB_(out) (n)) in the same order.

The phase and magnitude adjustments may be computed in a similar fashionas described previously.

It should be noted that some of the digital signal processing operationsdescribed herein may be performed in software, hardware, or acombination of the two. Such software may be run on, for example, adedicated or general purpose machine, such as a computer system, whilethe hardware may comprise, for example, dedicated logic, circuits, etc.Also, although the above describes an embodiment that performs digitalprocessing in the polar coordinate domain, the processing could beperformed in the I and Q domain.

Accordingly, a technique for amplifying signals with complex amplitudeand/or phase modulation is achieved by separating the signal into itsenvelope and angle modulated components, applying additional phaseand/or pulse modulation to two or more copies of the angle modulatedcomponent, amplifying these components in separate constant amplitudeamplifiers to high power levels, and combining the results toreintroduce the envelope modulation on the output, particularly using acombination of amplifiers switching, pulse modulation and phasemodulation. Advantageous results are gained from the constant amplitudesignals being amplified, since little or no additional phase oramplitude distortion is introduced by the high power linear amplifier.Furthermore, an embodiment is described which uses digital signalprocessing to generate the precision drive signals required by eachamplifier.

Amplification techniques described herein provide numerous advantages.For instance, the amplification technique(s) provides for high poweredlinear amplification of signals possessing considerable amplitude andphase modulation with high fidelity and low distortion, which may beextended to composite signals comprising two or more modulated carriers.The high power linear amplification may be achieved using multiplesimple low cost amplifiers operating at or near saturated power.

Additionally, in one embodiment, the present invention is advantageousin that it achieves the high power linear amplification using digitalsignal processing techniques to implement the amplitude reconstructionmodulation. The digital implementation of the amplitude reconstructionmodulator inherently provides greater precision than can be achievedusing analog techniques and, thus, enables obtaining the accuracy andcalibration needed to meet distortion requirements. Furthermore, thedigital signal processing can be applied as an adjunct to current classC amplifiers (for example, such as those used in current AMPS BASEstations) for linearization purposes while also amplifying the amplitudeand/or phase modulated signals using these amplifiers.

In one embodiment, the present invention allows for striking a balancebetween bandwidth and linearity, where the greater the bandwidth, themore linearity that may be obtained.

Moreover, the high power linear amplification is advantageous in that itachieves such amplification using a closed loop calibration techniquewhich compares the delayed input carrier against the output amplifiedcarrier for differences in a give instant of time and adjusts theamplitude reconstruction modulation process to remove (or null) thedifference. Also in one embodiment an amplifier linearization techniqueis provided that uses amplitude and phase controls in a polarcoordinates domain.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Many other variations arepossible. For example, pulse modulation can be implemented by use ofdifferential phase modulation eliminating the need for switch and pulsedrive signals to the amplifier. Therefore, references to details ofvarious embodiments are not intended to limit the scope of the claimswhich in themselves recite only those features regarded as essential tothe invention.

Thus, a linear amplification technique has been described.

I claim:
 1. An apparatus for amplifying an input signal comprising:a signal separator to separate the input signal into an amplitude component and a phase modulated component; a divider coupled to the signal separator to generate drive signals representing a plurality of replicas of the phase modulated component; a plurality of parallel paths coupled to the divider to receive the drive signals, wherein at least two of the plurality of paths comprising a switch and a phase modulator and each of the plurality of paths comprises an amplifier producing an amplified signal; a combiner coupled to sum outputs of amplifiers of the plurality of parallel paths to produce an amplified output; and an amplitude reconstruction controller coupled to the signal separator and said at least two paths to control each switch and phase modulator contained therein, wherein the amplitude reconstruction controller induces amplitude modulation to appear at the combiner output by introducing into the plurality of paths a combination of amplifier switching, pulse duty cycle modulation and phase modulation.
 2. The apparatus defined in claim 1 wherein said each switch and phase modulator are used separately or in combination depending on dynamic range to extend the dynamic range beyond that attainable by each used alone.
 3. The apparatus defined in claim 1 wherein amplitude modulation of the amplified output of the combiner matches amplitude modulation of the input signal.
 4. The apparatus defined in claim 1 wherein the signal separator comprises:a limiter to generate the phase modulated component in response to the input signal; and a detector to generate the amplitude component in response to the input signal.
 5. The apparatus defined in claim 1 wherein the detector comprises an envelope detector.
 6. The apparatus defined in claim 1 further comprising an calibration unit coupled to the output and the amplitude reconstruction modulator to generate at least one calibration signal in response to the output, wherein the amplitude reconstruction modulator controls said each switch and phase modulator based on said at least one calibration signal.
 7. The apparatus defined in claim 6 wherein the calibration unit generates said at least one calibration signal based on a comparison between the amplified output and a delayed version of the input signal, said at least one calibration signal causing the amplitude reconstruction modulator to compensate for amplitude and phase imbalances in the plurality of paths.
 8. The apparatus defined in claim 7 wherein said at least one calibration signal causes a fixed amplitude and phase correction to be applied to each of the plurality of parallel paths.
 9. The apparatus defined in claim 1 further comprising an isolator coupled to the amplified output and a filter coupled to the isolator to absorb distortion products and out of band harmonics at the combiner output.
 10. The apparatus defined in claim 1 further comprising a phase modulation combining module having a pair of adders to add and subtract phase modulation generated by the amplitude reconstruction controller to the phase modulation component first and second half channels respectively.
 11. The apparatus defined in claim 10 further comprising a calibration module to generate phase calibration signals, and wherein the phase modulation combining module further comprises a plurality of adders to add the phase calibration signals to the outputs of the pair of adders.
 12. A method of amplifying an input signal comprising the steps of:removing amplitude modulation from the input signal; dividing the angle modulation component into a plurality parallel paths having amplifier stages, wherein at least two of the plurality of parallel paths have a switch and a phase modulator preceding the amplifier stage of the path; inducing amplitude modulation into the plurality of paths by performing amplifier switching, pulse duty cycle modulation and phase modulation; and combining outputs of the amplifier stages in each path to regenerate an output signal having the amplitude and phase modulation of the input signal.
 13. The method defined in claim 12 further comprising the step of amplifying signals in the plurality of paths in a constant amplitude mode.
 14. The method defined in claim 12 further comprising the step of applying phase modulation before amplifiers in the plurality of paths.
 15. The method defined in claim 12 further comprising the step of modulating signals on all the plurality of paths with the same duty cycle modulation.
 16. The method defined in claim 12 further comprising the step of applying amplifier channel switching differently for amplifiers of at least two of the plurality of paths.
 17. The method defined in claim 12 wherein the amplitude modulation induced to appear at the output signal matches the amplitude modulation of the input signal.
 18. The method defined in claim 12 wherein the phase modulation induced to appear at the output signal matches the phase modulation of the input signal.
 19. The method defined in claim 12 further comprising the step of feeding back the output to provide calibration to compensate for mismatch of the plurality of paths.
 20. An apparatus for amplifying a signal comprising:means for separating an input signal into an amplitude modulation component and an angle modulation component; means for dividing the angle modulation component into a plurality parallel paths having amplifier stages, wherein at least two of the plurality of parallel paths include a switch and a phase modulator; means for inducing amplitude modulation into the plurality of paths by performing amplifier switching, pulse duty cycle modulation and phase modulation; and means for combining outputs of the amplifier stages in each path to create an output signal.
 21. The method defined in claim 20 wherein the amplitude modulation induced to appear at the output signal matches the amplitude modulation of the input signal.
 22. The method defined in claim 20 further means for feeding back the output to provide calibration to compensate for mismatch of the plurality of paths.
 23. An amplifier to amplify an input signal to high power levels with low distortion, said amplifier comprising:means for separating the input signal into its envelope and phase modulated components; means for providing replicas of the phase modulated component to each of a plurality of parallel channels; means for imparting pulse and/or phase modulation in each of the plurality of parallel channels; means for generating switch and phase modulation for amplitude reconstruction based on a function of the signal envelope, wherein the means for generating supplies signals to each of the plurality of parallel channels; means for controlling amplitude reconstruction based on a combination switched amplifier, pulse duty cycle, and differential phase modulation; a plurality of amplifier means for amplifying the phase modulated component in each or any combination of the plurality of parallel channels; and combining means for summing the output of the plurality of amplifiers means.
 24. The amplifier defined in claim 23 wherein the combining means comprises a power combiner.
 25. The amplifier defined in claim 23 further comprising means for absorbing out of band harmonics and distortion products at the output of the means for combining.
 26. The amplifier defined in claim 25 wherein the means for absorbing comprises an isolator and a filter.
 27. The amplifier defined in claim 20 wherein the means for absorbing approximately minimizes the out of band harmonics and distortion products.
 28. The amplifier defined in claim 18 further comprising a calibration module having:means for selecting samples of both the input and output signals; means for generating AM/AM and AM/PM response data from the samples; means for generating fixed phase and amplitude calibration signals for each of a plurality of parallel amplifier channels based on analysis of the AM/AM and AM/PM response data; means for imparting phase, amplitude and/or pulse width correction into one or more of the plurality of parallel amplifier channels; means for performing calibration to null residual phase and amplitude imbalances that exist in the plurality of parallel amplifier channels.
 29. The amplifier defined in claim 23 wherein the plurality of parallel amplifier channels comprise fully saturated RF amplifiers.
 30. The amplifier defined in claim 29 wherein the fully saturated RF amplifiers comprise Class C amplifiers.
 31. An amplifier comprising:means for performing analog to digital conversion to sample an analog input and calibration signals centered at an intermediate frequency (IF); means for converting sampled the signals into complex baseband signals; means for performing rectangular to polar conversion on the complex baseband signals to generate amplitude and phase modulated components; means for computing pulse and/or phase modulation for reconstituting amplitude modulation based on vector recombination of multiple amplifier outputs; means for generating and combining amplitude and phase calibration signals with amplitude reconstruction modulation signals to generate signals to drive a plurality of amplifier channels; means for performing polar to rectangular conversion on signals from each of the amplifier channels to reconstitute I and Q signals; means for converting complex baseband signals on each of the signals from the amplifier channels into real signals; and means for converting the digital signal on each of the amplifier channel signals into analog signals centered at the IF.
 32. The amplifier defined in claim 31 further comprising means for decimating the complex baseband signals prior to rectangular to polar conversion.
 33. The amplifier defined in claim 31 further comprising means for interpolating data corresponding to the real signals prior to conversion of the digital signals of the amplifier channels into analog signals.
 34. An apparatus for amplifying an input signal comprising:an analog-to-digital converter (ADC); a digital signal processor coupled to the ADC, where the digital signal processor comprises: a rectangular to polar converter coupled to the ADC to convert the rectangular coordinate in-phase (I) and quadrature-phase (Q) input into a polar coordinate amplitude and phase format, a phase modulation combining module to provide phase modulation to the polar coordinate phase component, and a plurality of polar to rectangular converters coupled to the phase modulation combining module to convert amplitude and phase components from polar coordinates into rectangular coordinates; and a plurality of amplifier modules, each being coupled to one of the plurality of polar to rectangular converters; a combiner coupled to sum outputs of amplifiers of the plurality of amplifier modules to produce an amplified output.
 35. The apparatus defined in claim 34 wherein each of the amplifier modules comprises a digital analog converter (DAC) coupled to an output of one of the plurality of polar to rectangular converters, and an amplifier coupled to the output of the DAC.
 36. The apparatus defined in claim 34 wherein each of the channel amplifier modules further comprise:a mixer coupled to the output of the DAC to mix a signal output from the DAC with a local oscillator frequency; and a filter coupled to the output of the mixer, wherein the output of the filter is coupled to the input of the amplifier of said each of channel amplifier modules.
 37. The apparatus defined in claim 34 further comprisinga digital down converter coupled to the input signal; a plurality of digital up converters coupled to the plurality of polar to rectangular converters to upconvert signals received from the polar to rectangular converters.
 38. The apparatus defined in claim 34 further comprising a plurality of output equalizers coupled between the plurality of polar to rectangular converters and the plurality of digital up converters.
 39. The apparatus defined in claim 27 further comprising an input equalizer coupled between the digital down converter and the rectangular to polar converter. 